In a telecommunication system, for example, a line link having a pair of coaxial cables for the sending of messages in opposite directions generally includes a number of repeaters whose amplifiers may receive direct or alternating current from one of the terminal stations served by that link. In some of these systems the energizing current is transmitted, under normal operating conditions, in a balanced manner with reference to ground over the line loop formed by the two conductors and the load constituted by the components of the repeater, the loop being insulated from ground at all points whereby the load is held at a floating potential. As long as the symmetry of the circuit is not disturbed, there exists only a small potential difference between the load terminals and ground so that maintenance personnel working at the repeater will not be imperiled.
If an unbalance develops along the line, e.g. on account of a ground fault, that potential difference may become dangerous. This can also be the case when the load is accidentally open-circuited so that a large voltage difference exists across its terminals.